It has long been known that inorganic coatings can be applied to a glass surface by contacting a hot glass surface with a metal bearing compound which decomposes to form what is believed to be a metal oxide layer on the surface of the hot glass. Processes of this type were disclosed by Lyle in U.S. Pat. No. 2,375,482 for imparting an iridescent finish to glass.
More recently, processes of this type have been adapted for use in protective coatings for glass containers, particularly, beverage bottles and other similar containers. In these processes glass containers, such as glass bottles, while still hot from the bottle-forming equipment and before passage through the annealing lehr, are treated with a thermally-decomposable metal compound, such as stannic chloride or titanic chloride, under conditions such that a thin coating is formed on the container surface. This coating usually is thinner than that taught by Lyle, and serves to bond a lubricious organic polymer or wax coating applied to the container surface after the container exits from the lehr. This combination of metal oxide "hot end" coating and organic "cold end" coating has been found useful in improving the scratch resistance and lubricity of glass containers.
Although this combined coating has been found useful, the generally employed methods of applying a hot end coating have several drawbacks. Generally anhydrous compounds, especially anhydrous stannic or titanic chloride, were employed which led to numerous problems. First, it was necessary to entrain the metal halide vapors in a dry air stream, which normally is accomplished by bubbling dry air through a liquid metal halide. If moisture should be introduced into the resulting air stream, a precipitate results which may inhibit the flow of entrained metal halide vapors thereby causing a reduction in the amount of coating material supplied to the application chamber. Furthermore, reactions between the metal halide and water in the atmosphere or the pyrolysis of the metal halides results in a reaction product which is corrosive to the equipment employed in the production of glass articles. The reaction product also is noxious in odor, which often creates undesirable working conditions in the hot end of glass manufacturing plants. Other reaction products are particulate materials which are difficult to capture.
Next, it is difficult to ensure formation of a uniform hot end coating when anhydrous metal halide fumes are used because they can react with moisture in the atmosphere before contacting the glass surface. The results are non-uniform coating thickness and poor bottle-to-bottle reproducibility. Moreover, the loss of metal halide through such a reaction seriously reduces the efficiency of the use of the expensive metal halide reagent.
Finally, it is essential to prevent formation of a metal oxide coating on the finish, or mouth, of bottles particularly to avoid corrosion of bottle caps and high removal torques. This control is difficult to achieve with air streams containing entrained metal halide vapors, especially with the so-called "stubby" beer bottles which are commonly employed today.
To some extent, these problems can be avoided through use of sprays of aqueous or alcohol solutions of tin halide hydrates, as is disclosed in U.S. Pat. No. 3,819,346. However, such solutions are highly acidic and special equipment are required to handle the corrosive liquids. These coating materials also create noxious and corrosive reaction products upon their pyrolytic decomposition.
Not only have inorganic compounds, such as metal halides, been employed, but considerable effort has been devoted to the use of organo-metallic compounds. For example, Deyrup, in U.S. Pat. No. 2,831,750 issued Apr. 22, 1958, discloses applying vapors of a metallo-organic compound such as tetraisopropyl titanate to hot glass (450.degree.-600.degree. C.). According to Deyrup the corresponding inorganic compounds are either too heat stable or insufficiently volatile without decomposition to be suitable for such use. Subsequently, Gray et al, in U.S. Pat. No. 3,004,863 described a process in which aqueous solutions of certain aqueous acid-soluble titanates were applied to glass at room temperature and the glass was thereafter annealed, at which time the titanium oxide coating was formed. Still more recently, Green et al, in U.S. Pat. No. 3,667,926 issued June 6, 1972, disclosed a process wherein an aqueous solution of a water-soluble titanium composition was sprayed onto hot glass. As was the case with Deyrup and Gray et al, Green et al employed solutions of organo-titanium compounds.
U.S. Pat. No. 3,387,994 to Dunton et al discloses that an improved process for applying a titanium coating to glass comprises spraying a heated glass surface with an inert, non-aqueous organic solvent solution of a titanium ester complex. According to the patent, the ester complex is the reaction product of one mole of a tetraalkyl titanate, e.g., tetraisopropyl titanate, and one mole of a chelating agent, e.g., acetylacetone. Also according to the patent, the nature of the solvent can vary widely, and includes liquid hydrocarbons and halogenated derivatives and alcohols. Isopropyl alcohol is preferred, and was used in all of the Examples. It has been found, however, that the preferred process of Dunton et al using, e.g., tetraisopropyl titanate chelated with acetyl acetonate, in an alcohol solution does not yield a commercially practicable process.
As was the case with the inorganic compounds, the use of organo-metallic compounds was not entirely satisfactory. For example, the vapors of anhydrous organo-metallic compounds, such as tetraisopropyl titanate, react with moisture in the atmosphere and decompose. The results are non-uniform coating thicknesses and poor bottle-to-bottle reproducibility. When trying to spray liquid anhydrous organo-metallic compounds, the liquid reacts with moisture in the atmosphere and forms a solid deposit which tends to plug spray nozzles. Furthermore, the techniques previously employed to apply such compounds typically form finely dispersed particles which are entrained in the air stream, and are difficult to remove from it in a simple and economic fashion.